The present invention relates to thermal transfer media used in thermal transfer printing and coating formulations used to produce them. Images are formed on a receiving substrate by heating extremely precise areas of a thermal transfer medium with thin film resistors. This heating of the localized areas causes transfer of ink from the thermal transfer medium to the receiving substrate.
Thermal transfer printing has displaced impact printing in many applications due to advantages such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal transfer printing includes the following patents and publications.
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, U.S. Pat. No. 4,463,034, issued to Y. Tokunaga et al. on Jul. 31, 1984, U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, U.S. Pat. No. 4,983,446, U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, U.S. Pat. No. 5,240,781, issued to Obatta et al. U.S. Pat. No. 5,328,754 issued to Yugama et al., EP 0111004 published in June of 1984, Patent Abstracts of Japan, Vol. 17, No. 286 (M-1422), published on Jun. 2, 1993 and JP 05 016533, filed by Ricoh K. K., which published on Jan. 26, 1993.
Many attempts have been made to provide high integrity thermal transfer printing which is resistant to scratching and smearing, some of which are described above. Examples are U.S. Pat. Nos. 5,128,308 and 5,248,652 issued to Talvalkar.
U.S. Pat. Nos. 5,952,098 and 6,172,142, issued to Lorenz described thermal transfer media which contain epoxy resin binder and crosslinker in separate phases and coating formulations which produce them.
Government legislation has made the reduction in the use of organic solvents increasingly attractive. As such, it is desirable to use water as a carrier solvent for coating formulations.
The present invention provides aqueous coating formulations for preparing thermal transfer layers of thermal transfer media and also thermal transfer media obtained with these aqueous coating formulations.
One embodiment of this invention is an aqueous coating formulation which forms a thermal transfer layer of a thermal transfer medium having a softening point below 200xc2x0 C. This coating formulation comprises an aqueous emulsion of at least one thermoplastic resin and/or wax and at least one epoxy curing agent which initiates crosslinking with an epoxy resin and is coemulsified with the one or more thermoplastic resins and/or waxes. The coating formulation comprises an aqueous liquid which does not solubilize the epoxy curing agents or the thermoplastic resins or waxes. Each of the epoxy curing agents, thermoplastic resins and waxes has a softening point below 200xc2x0 C. The thermoplastic resins and waxes are solid at 20xc2x0 C. and the epoxy curing agent is either solid at 20xc2x0 C. or encapsulated in a wax or thermoplastic resin which is solid at 20xc2x0 C.
Another embodiment of this invention is an aqueous coating formulation which forms a thermal transfer layer of a thermal transfer medium having a softening point below 200xc2x0 C. which comprises a combination of a) an aqueous emulsion of at least one thermoplastic resin and/or wax coemulsified with at least one epoxy curing agent which initiates crosslinking with an epoxy resin and b) an aqueous dispersion of at least one epoxy resin. This aqueous coating formulation comprises an aqueous liquid which does not solubilize the epoxy curing agents, thermoplastic resins, waxes or the epoxy resins. The epoxy resins, epoxy curing agents, thermoplastic resin and waxes each have a softening point below 200xc2x0 C. so as to melt mix at a temperature in the range of 50xc2x0 C. to 250xc2x0 C. The thermoplastic resins and waxes are solid at 20xc2x0 C. and the epoxy curing agent is either solid at 20xc2x0 C. or encapsulated in a wax or thermoplastic resin which is solid at 20xc2x0 C.
In another aspect of this invention, there is provided thermal transfer media obtained from the aqueous coating formulations of this invention. One embodiment of these thermal transfer media comprises a flexible substrate and a single thermal transfer layer which has a softening point below 200xc2x0 C. obtained from an aqueous coating formulation of this invention. This single thermal transfer layer comprises at least one epoxy resin, at least one curing agent which initiates crosslinking with the epoxy resin and at least one thermoplastic resin and/or wax, where each epoxy curing agent is dispersed within the one or more thermoplastic resins and/or waxes and separated from each epoxy resin so as to not react without melt mixing. Each of the epoxy resins, epoxy curing agents, thermoplastic resins and waxes are solid at 20xc2x0 C. and have a softening point below 200xc2x0 C. so as to melt mix at a temperature in the range of 50xc2x0 C. to 250xc2x0 C.
Another embodiment of the thermal transfer media of this invention comprises a flexible substrate and two thermal transfer layers, each having a softening point below 200xc2x0 C. The first layer comprises at least one epoxy resin and the second layer is obtained from an aqueous coating formulation of the present invention comprising at least one epoxy curing agent which initiates crosslinking with the epoxy resin, and at least one thermoplastic resin and/or wax, wherein each epoxy curing agent is dispersed within the thermoplastic resin and/or wax and is separate from the layer of epoxy resin so as not to react with the epoxy resin without melt mixing. Each of the epoxy resins, epoxy curing agents, thermoplastic resins and waxes are solid at 20xc2x0 C. and have a softening point below 200xc2x0 C. so as to melt mix at a temperature in the range of 50xc2x0 C. to 250xc2x0 C. The thermoplastic resins and waxes are solid at 20xc2x0 C. and the epoxy curing agent is either solid at 20xc2x0 C. or encapsulated in a wax or thermoplastic resin which is solid at 20xc2x0 C.